1. Related Applications
This invention improves the performance of the refrigerator thermal storage tank in my prior application No. 897,274 filed on Jun. 11, 1992, which was subsequently issued as U.S. Pat. No. 5,237,832 on Aug. 24, 1993.
2. Field of Invention
The present invention relates to an improved refrigerant evaporator coil for removing heat from static solutions. More particularly, the coil is especially well suited for use within phase change solution containment tanks in systems which utilize azeotrope and non-azeotrope refrigerants.
3. Discussion of Prior Art
The difficulty in attaining improved heat exchanger efficiency when using a static phase change solution within the confines of a thermal containment tank is well known. The need to maximize the surface area which is in contact with the solution conflicts with the goal of containing as much solution as possible within a minimum amount of space. Increased coil volume reduces the space available within the tank for the phase change solution itself. Additionally, since the solution being cooled is static, the coil configuration must be such that it is equally spaced throughout the tank to provide even cooling. Also, both the evaporator and its support structure must withstand the forces exerted by the repeated expansion and contraction of the surrounding phase change solution as it changes state.
In a typical example of the prior art exemplified by Kleist (U.S. Pat. No. 2,859,945), a single tube is bent so as to allow a maximum length to fit within the space allowed. To increase the total surface area of the coil, a tube of either larger diameter or longer length must be used. Due to mechanical limitations in the tube bending process, selection of a larger diameter tube increases the minimum bend radius thereby decreasing the length of the tubing which will still fit within the same overall dimensions. The result is no net increase in the total surface area of the coil. An additional restriction on increasing the tube diameter is the requirement to maintain sufficient refrigerant velocity to provide adequate oil return.
A similar coil utilizing smaller diameter tubing with increased length is shown by Rodth (U.S. Pat. No. 4,291,546). While providing increased surface area, this approach suffers two major limitations and deficiencies. One is the increased pressure drop which occurs when the refrigerant flows through extended lengths of smaller diameter tubing. This pressure drop reduces, not only the performance of the coil, but the volumetric efficiency of the compressor as well.
The second, related limitation, lies in the increased temperature variation as the refrigerant travels the length of the coil. This temperature variation leads to uneven cooling within the tank and difficulty in maintaining a stable superheat. This is particularly problematic with the new environmentally friendly non-azeotrope refrigerants. These refrigerants are comprised of a blend of several refrigerants which, in an evaporator coil, each boil off at a different rate. This variation is known as temperature "glide" and greatly exacerbates the tendency of evaporator coils which are immersed in phase change solutions to freeze the material nearest the evaporator inlet first. The net effect of this inherent temperature glide is to exaggerate temperature variation throughout the tank such that the expansion valve superheat settings must be so high as to negatively effect the efficiency of the entire system.
Further attempts to increase evaporator surface area and, consequently, evaporator efficiency, have taken the form of the addition of metal fins and similar enhancements of the outer wall of the tubing. Such enhancements have been found to be very helpful in a gas (air) to liquid (refrigerant) heat exchange, such as is found with an air conditioning coil. However, in the liquid/solid (static phase change solution) to liquid (refrigerant) heat exchange with which we are concerned, both the inner and outer surface areas of the evaporator tubing must be increased simultaneously to improve performance. When only one surface area is increased the limiting factor simply becomes the other surface. A variation on this approach can be found in the cooling coil and related support structure by Horton (U.S. Pat. No. 4,356,708) which does little to aid in the removal of heat from the state change solution since only the outer surface of the evaporator tube is enhanced.
It is therefore recognized that one effective way to increase the usable surface area for such evaporators is through the use of multiple small diameter tubes of shorter length. The aggregate total of which provides greater inner and outer surface area while still maintaining refrigerant velocity and minimum pressure drop. An evaporator incorporated in the invention of Fischer (U.S. Pat. No. 4,735,064) teaches such an approach but fails to resolve two problems persistent in the prior art of this type. The first problem stems from difficulty in equally distributing refrigerant to each of the coils. Refrigerant fed to the coil from a linear header as advised overfeeds the coil(s) at the end of the header and underfeeds those at the beginning. The obvious solution of connecting all tubes at a central point proportionately increases the length of each evaporator coil as its distance from this point of origin increases. The steadily escalating length of each individual tube would further increase the second major defect of these evaporators which is, asymmetrical freezing of the state change solutions. Incoming refrigerant feeds all tubes from a common end of the tank thereby chilling, and eventually freezing, this end well before that end at which the tubes exit.
A refrigerant heat exchanger which provides a limited solution to these problems is shown by Bartlett (U.S. Pat. No. 4,995,453). Designed primarily for use with fins as a refrigerant to air heat exchanger, it incorporates a single pressure drop minimizing tube which branches into two separate circuits at the point of the first tube bend. This design provides an improvement in cross-feed pattern and pressure drop when only two circuits are required. However, in expanded embodiments requiring more than two circuits, such as those in large, multi-layered coils, its advantages become increasingly limited and eventually unsuitable since routing complexities require circuits of unacceptable variations in length. An additional disadvantage in this invention is the mid-coil connections between the pressure drop minimizing tube and its circuits which present a corrosion point if the coil assembly is immersed in a static phase change solution.
In addition to those items specifically mentioned above, the prior art has been found to suffer from one or more of the following disadvantages;
a. High production cost which requires the use of expensive tube forming and bending equipment.
b. The use of dissimilar metals and other materials incompatible with refrigerants, oils and/or a wide variety of static solutions.
c. The use of soldered, welded, brazed, pressed, screwed or other connections within the phase change solution containment tank which are susceptible to leakage and corrosion.
d. Unequal spacing throughout the static solution containment tank.
e. Difficulty or inability to incorporate a suitable support structure within the static solution containment tank.
f. Difficulty or inability to be easily modified to accommodate systems of greater or lesser capacity.
g. An inability to consistently withstand the physical stress exerted by the phase change solution.